Constant Pressure and Constant Volume [ENDORSED]

[705960571]

Constant pressure and constant volume are two conditions that are mentioned frequently in the topic of thermochemistry, however, I am getting a little confused about what to assume under these conditions.

For ∆H = q is this only for constant pressure or also for constant volume?

What conditions do we use the ∆H = ∆U + ∆ngas RT equations? Is this when volume changes because gas is produced?

Bomb calorimeters: There are constant volumes but not constant pressure correct?

If Volume is constant then would work done by the system always be 0?

Is work done by the system always 0 for solids and liquids since volume doesn't change?

Coffee Cup Calorimeters: These are constant pressure but not constant volume right?

[Chem_Mod]

∆H = q_p is the definition of enthalpy.

Remember in class I discussed that solids and liquids have very small volume changes on heating and therefore one can ignore expansion work. Therefore q_p and q_v are essentially the same value and equal to delta U (as w = 0):

∆U = q + w = q

For gasses, heating them at constant pressure means they have to expand, and therefore do work of expansion.

∆U = q_p + w = ∆H - ∆n(gas) RT

For gasses, heating them at constant volume: delta U = q + w = q

Make sure to ask your TA to go through examples in office hours.

Your answers are correct:

What conditions do we use the ∆H = ∆U + ∆n(gas) RT equations? Is this when volume changes because gas is produced? YES

If Volume is constant, then would work done by the system always be 0? YES

Is work done by the system always 0 for solids and liquids since volume doesn't change? CORRECT

Coffee Cup Calorimeters: These are constant pressure but not constant volume right? CORRECT

Took me a long time to figure out how to answer so many questions clearly.